This proposal requests support for the 15th year to study the lineage stages of osteogenic development from osteoprogenitor cell to expressive osteoblast and osteocyte and to establish and characterize each individual and discrete lineage stage. This proposal is built on our previous funded grant application in which unique markers for osteogenic cells and the morphology of long bone formation were emphasized Substantial progress has been made on these previous Specific Aims and provides the experiential base for attempting to continue to develop a battery of markers, principally monoclonal antibodies, for each osteogenic lineage stage. These markers will be used to morphologically identify specific lineage stage cells during lone bone formation and will also be used in an attempt to purify lineage stage-specific cells in order to study the sensitivity of these cells to various mitogens and hormone controlling factors. Eventually, it is our goal to identify in vivo repositories of osteoprogenitor cells so that they may be appropriately manipulated to provide large amounts of osteo-reparative cells. To accomplish these aims, three separate projects are proposed: The first is to generate monoclonal antibodies to the full range of osteogenic cells. Five monoclonal antibodies are now in hand and these will be further characterized. The goal will be to obtain additional monoclonal antibodies to further identify lineage subsets as well as osteoprogenitor cells. Project II involves the purification of lineage-specific cells principally by using monoclonal antibodies to purify specific cells. Project III proposes detailed studies of comparative osteogenesis involving osteogenic lineages at various sites of bone formation. In addition, different aged specimens as well as those undergoing repair and/or ectopic osteogenesis will be compared to determine if the lineage progression observed under these conditions is comparable to that observed during first bone formation. The successful completion of these experimental goals will result in a better understanding of the progressive changes that osteogenic cells undergo from osteoprogenitor cells to secretory osteoblasts or osteocytes. In addition, the molecules which play predominant roles in controlling lineage progression will be identified and allow schemes to be proposed for the manipulation of these cells to combat various osteo-diseases.